The Instrument Landing System (ILS) localizer provides azimuth guidance to aircraft on final approach to landing, by radiating guidance signals from an antenna array located at the stop end of the runway and disposed perpendicular to the runway. Early localizer arrays used antenna elements that were nearly omnidirectional in a horizontal plane, in order to generate both a front and back course, and to transmit all-around "clearance" (full scale right or left) signals. Courses radiated by early installations were sometimes seriously disturbed by localizer signals reflected from buildings, parked and taxiing aircraft, and aircraft flying over the localizer array. Partly for the purpose of reducing these disturbances, the requirements for a back course and all around clearance were dropped, thus admitting the use of arrays whose radiation patterns were confined to a relatively small angular region on either side of the runway centerline. Reduction in disturbance of the localizer course by aircraft flying over the localizer array was achieved by the use of large corner reflector screens, or with end-fire antenna elements. The latter have come into widespread use in modern localizer antenna arrays. The directivity of the end-fire element, in addition to reducing overflight interference, also practically eliminates the back course, and permits the use of low power solid state transmitters.
Even with relaxed requirements for clearance and back course, and the employment of arrays using end-fire elements, it proved difficult or impossible to meet specifications for localizer course straightness at some sites, because the angular width of the localizer radiation pattern was still not narrow enough to avoid the illumination of reflecting objects by localizer signals which were subsequently reflected into the approach path and produced bends in the localizer course. If the radiation pattern is to be made narrower, antenna theory teaches that the "aperture" (overall length perpendicular to the runway) of the localizer array must be increased. Any increase in aperture must be accompanied by an increase in the number of elements, according to conventional array design procedure, to avoid the appearance of spurious radiation which will occur if the spacing between antenna elements exceeds one wavelength, such spurious radiation being often referred to in the literature on optical or antenna design as a "interferometer lobe" or "grating lobe" . Antenna theory teaches that if main beams of the array are directed toward angles .+-..theta..sub.o from a line perpendicular to the array and if the spacing between antennas is S, then interferometer lobes will appear in the array factor at angles .+-..psi. as determined from the equation .psi.=sin.sup.-1 (.lambda./S-sin .theta..sub.o), array factor being conventionally defined as the radiation pattern of the array that would exist if all elements of the array radiated isotropically. In an array designed for the narrowest possible radiation pattern consistent with course width requirements, these considerations lead to a design employing a relatively large number of elements, which is undesirable from the standpoints of cost, reliability, and maintenance. It is the object of the present invention to reduce the number of elements required in a wide aperture localizer array, by using to advantage the directivity of end-fire antenna elements that would be employed in any case to minimize overflight interference and reduce required transmitter power.